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25. The answer is C. (Keratoacanthoma) Keratoacanthoma (KA) is a rapidly growing hyperkeratotic nodule with a central keratin plug. A KA typically develops over 3 to 6 weeks, in contrast to the slow growth of typical SCCs over months to years. KAs occur most commonly in areas of sun-damaged skin. KAs are clinically and histologically indistinguishable from well-differentiated SCC. The etiology of KA is not certain, however; human papillomavirus DNA has been found in some cases. KAs have also occurred in skin soon after radiation therapy. Additionally, there are syndromes of multiple KAs developing over years. There is controversy regarding whether KAs are malignant or benign. Although they resemble SCCs histologically, most spontaneously regress with scar formation. There have been reported cases of invasive KAs, some with metastases, leading many experts to consider all KAs a form of SCC and treat them as such. Because of the uncertainty malignant potential of KAs, most are treated as well-differentiated SCC.
Shaw JC. Overview of nonmelanoma skin cancers. Up to Date, version 14.1. Accessed 5/3/06.
